In praise of the Tevatron

Fermilab's two accelerator rings, as seen from the air. Credit: {link url="http://en.wikipedia.org/wiki/File:Fermilab.jpg"}Fermilab, Reidar Hahn{/link}

Tomorrow, the Tevatron particle accelerator at Fermilab will shut down. The end will be no song and dance: the accelerator operators will simply stop putting new protons and antiprotons into the machine. The last few particles will whiz around the accelerator until the number of collisions per second drops below a useful level, after which time the remaining particles will be diverted to a metal target. The process is the same as that used for annual maintenance shutdowns, but this time there will be no going back. By the end of December, the thousands of superconducting magnets that surround the accelerator tunnel will have been warmed up from their close-to-absolute-zero temperature before being removed, and the rest of the accelerator will have been cleared of gases and fluids, and dismantled. Its a rather understated end to 26 years of smashing particles together in the name of science.

From the placing of the first magnet to those final few collisions, the Tevatron has been a major engineering achievement and a source of plenty of new science for particle physicists to sink their teeth into. Even after the shutdown, scientists working on Fermilab's CDF and DZero experiments will carry on analysing Tevatron data and in all likelihood will end up publishing at an even higher rate than they did when the accelerator was running, for a couple of years at least.

In honour of the shutdown, the nice people Fermilab have produced an interactive timeline of the Tevatron's history. You can see the timeline here. And for a bit more detail on one of the most important discoveries made there, read on...

March 2, 1995

Just before the 10th anniversary of the first proton-antiproton collisions witnessed by the CDF collider detector, both the DZero and CDF experiments submitted papers to Physical Review Letters detailing their discovery of a new particle — the top quark. The discovery was announced in seminars at Fermilab and a press release sent out on the same day.

Thinking that quarks should come in pairs, particle physicists had been hunting for the top quark since the discovery of its smaller partner, the bottom quark, at Fermilab in 1977. The CDF experiment saw hints of the top quark in 1994, but there wasn't quite enough data to say for certain that it existed. Once more data had been collected and analysed, and the DZero experiment had conducted its own, independent investigation, a clear signal of the tops existence was found, allowing both groups to say for certain that they'd seen it.

The top quark is the heaviest fundamental particle we have ever observed, by a long way. It is 100,00 times heavier than the lightest quark — the up quark — and has about the same mass as a gold atom, though it is much smaller. Its mass is why it was the last quark to be discovered: the more massive a particle, the more energy is required to create it in a particle accelerator. Before the Large Hadron Collider, the Tevatron was the only accelerator in the world capable of reaching energies high enough to make the top quark.

To create a top quark, a proton and antiproton collide at nearly the speed of light. Sometimes, this results in the production of a top quark, which always comes with an antiquark. The pair don't hang around for long, and within a fraction of second they both decay into lighter particles. A chain of decays often results in a burst of particles that physicists call jets. Finding the signature of a top quark in the vast amounts of data would be near impossible, given its short lifetime, so instead physicists look for its decay products. A top-antitop pair decay into two W bosons (force particles that mediate the weak nuclear force) and two bottom quarks (the top's much lighter parter). In turn, one of the W bosons turns into a muon and neutrino pair and the other turns into an up and down quark pair. The up and down quarks then decay into jets, shortly followed by the bottom quarks that go the same way. So, the signature to look for when searching for top quarks is: one muon, one neutrino and four jets.

The CDF and DZero experiments had each seen approximately ten top-antitop quark pairs, which was enough for them to be sure that the events were not just something that looked like top quarks, but the real thing.

In the years after the discovery, scientists at Tevatron have been able to determine the top quark mass to high precision. This has not only helped to reinforce our knowledge of the standard model of particle physics, but also refine our ideas about the Higgs boson — the only particle in the standard model that has yet to be found. Now that the Tevatron is shutting down, it will be up to the LHC to continue the hunt for the Higgs.

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